Generally known are simple threaded plugs which can be screwed into a drainage opening made as a threaded bore in the wall of a container. It is considered to be a particular disadvantage of such threaded plugs that they do not allow quantitatively regulated draining of the liquid, since they either leave the drainage opening completely clear, or closed, at least to a large extent.
Also known are oil drainage fittings usually in the form of drain taps, which are screwed from the outside all the way into the drainage opening in the form of a threaded bore formed in the wall of the container. Since such oil drainage fittings project a long way out from the wall of the container, substantial additional fitting space must be left clear for them, apart from the fact that such fittings also risk being damaged. A further disadvantage is considered to be that there can exist a risk of leakage between the screwed-in fitting and the threaded bore.
From US 2009/0101440 A1 a drain valve for draining a liquid out of a container is known, which has an essentially cylindrical opening that passes through a thickened area of the container wall, with a threaded section at its outer end which in provided with an internal thread, this section being followed by an interior space with a larger diameter into which a duct opens, and with an inner section opposite the interior space which has a smaller diameter and opens into the container, the step between the interior space and the inner section serving as the seat for an associated end face of a valve element. The inner section is provided with an internal thread which receives an extension formed on the inner end of the valve element and is provided with an external thread, this extension acting essentially as a stopper but one that does not allow quantitatively regulated draining of the oil. For draining purposes the known valve element must first be unscrewed a great distance before it allows oil to pass through in a more or less uncontrolled manner. Moreover, this known design is expensive and elaborate to produce, since two respective aligned threaded sections with different thread diameters have to be made for both the valve housing and for the valve element.